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Re: neon protection





---------- Forwarded message ----------
Date: Wed, 8 Oct 97 16:09:24 EDT
From: Gary Lau  08-Oct-1997 1538 <lau-at-hdecad.ENET.dec-dot-com>
To: tesla-at-pupman-dot-com
Subject: Re: neon protection

>Date: Tue, 07 Oct 1997 22:19:07 -0400
>From: Someone <fox-at-netunlimited-dot-net>
>To: tesla-at-pupman-dot-com
>Subject: Re: neon protection
>
>i recently destroyed my first transformer and am looking for a way to
>protect it. ive seen protection circuits but they all seem a little
>vague. i dont have any accurate measuring devices so i cant make my own
>capacitors and chokes if they need to be precise.
>someone once told me that a flourescent tube ballast was just a big
>choke... could 2 of these on the hv terminals be used to keep it from
>burning out?
>
>thanks

The chokes used in a transformer protection network will develop many KV
across their terminals.  Unless you're using a single layer coil wound on
an insulating (not ferrite) form, special efforts must be made to keep the
two ends from arcing to each other.  Ferrite cores are useful, but only
if well insulated from the windings.  Flourescent tube ballasts were
never designed to support that kind of voltage and will surely arc
internally.  You may not be able to see, smell, or otherwise detect the
arcing, but it will, and the protection to the transformer will be lost.

The exact value of a choke is not important, as long as it is high
enough, 10 to 100 milliHenrys is what seems to be commonly used.  It's
not clear that a choke is of any benefit unless it is used in conjunction
with a bypass capacitor to ground at the transformer to form a low pass
filter.  Bypass cap values typically run in the 100-750 pF range.

Certainly the most important and easiest protective feature is a safety
gap from each HV side of the transformer to ground.  About 1/4" from
either side of the transformer to ground with rounded electrodes works
for me.

Gary Lau
Waltham, MA